Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include portable computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code, system data such as a basic input/output system (BIOS), and other firmware can typically be stored in flash memory devices.
Flash memory devices typically have a chip protection register that is implemented using non-volatile, flash memory cells. The register is used by the flash memory end-user to store data that needs to be protected from erasure. Once the data is stored and the lock bit is erased (i.e., a logic 1 state), the data cannot be changed.
FIG. 1 illustrates a typical prior art lock bit circuit. The lock bit 101 is erased after fabrication of the memory device but prior to shipment to the end-user. The connection to the erase circuit 103 is broken so that after the program circuit 104 programs the lock bit, it cannot be erased. The lock bit is typically a floating gate avalanche injection metal oxide semiconductor (FAMOS) cell. The lock bit 101 is read by cell read circuitry 107 that determines if the protection register is locked.
The lock bit may be surrounded and covered by circuitry and traces. Since the bit is erased with ultra-violet radiation, the surrounding circuitry may limit the amount of UV light that reaches the bit and, therefore, increase the chances that the bit will not be erased fully. The part may then ship with the bit already programmed so that the end-user cannot program the protection registers.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a more reliable chip protection register lock circuit.